Abstract
In this work, we present the hydrogen selective gas separation properties of the track-etched poly (ethylene terephthalate) (PET) membranes, which were functionalized with a carboxylic group. Also, Palladium (Pd) nanoparticles of average diameter 5 nm were deposited for a various time on pore walls as well as on the surface of carboxylated membranes. Effect of Pd nanoparticles binding with the increase of deposition time on gas separation and selectivity was studied. For the study of surface morphology of these composite membranes and the confirmation of Pd nanoparticles binding on the surface as well as on pore walls is characterized by scanning electron microscopy (SEM). The gas permeability of carboxylated membrane with increasing Pd deposition timing for hydrogen (H2), carbon dioxide (CO2) and nitrogen (N2) was examined. From the gas permeability data of H2, CO2 and N2 gasses, it was observed that these membranes have higher permeability for H2 as compared with CO2 and N2. Selectivity of H2/CO2 and H2/N2 improves with the increased Pd nanoparticles deposition time. These membranes have effective application in the field of hydrogen based fuel cell.
Introduction
Important scientific and technical challenges, can be achieved through the development of a hydrogen economy [1], [2], [3]. A large amount of hydrogen is consumed very speedily in refining industry as a fuel and augmentation of fuels are required for more improving environmental level [4]. There is huge requirement of hydrogen for the advanced material research and economical use in technology, in which hydrogen should be highly pure. Therefore, there are necessities to purify as well as separate high-quality hydrogen. For the production of high-purity gasses, membrane based processing is one of the best promising technologies. With the growing use of pure hydrogen as a fuel for the industrial applications, optimization of current membrane based technologies is required. Therefore, it is necessary to confiscate undesired elements and impurities from gasses and mixtures of solvents [5]. Membrane based gas separation has intrigued responsiveness/consciousness because of its fundamental restitution like energy efficient, cost effective, easy procedure over other traditional separation techniques, and environment friendly [6], [7], [8], [9].
There are different types of membranes like a polymeric membrane, metallic membrane, ceramic membrane, etc., which can be used for gas separation [10], [11], [12]. But polymeric track-etched membrane is one of the preferred for gas separation due to its tuneability of pore shape and also pore size depends on the choice of ion [13], [14], [15], [16]. In these membranes, tracks can be generated with the help of swift heavy ion irradiation process and afterward these tracks are converted into pores through chemical etching process [17], [18], [19]. Ion influence and energy of ion are that factor through which porosity can be altered in a controlled way during ion irradiation process [12], [18]. Because of outstanding mechanical stability, thermal resistance, as well as inertness track-etched PET membranes are frequently used In gas separation application [20]. Although, these membranes undergo low permeability as go for high selectivity [21]. The interaction ratio between the gas molecules with the membrane and their pore size, both are the basic fact which plays an important part in gas separation [22], [23].
With the growing interest in developing innovative approaches to functionalize the surface of the substance for regulating their surface properties according to requirements [24], [25], [26]. But it is difficult to control functionalization of porous material than the planer surface. Surface property of the membranes can be changed through the functionalization. When surface and pore walls both are chemically modified, this effect of functionalization leads significant influences in the gas separation. Functionalized membranes are more effective for depositing preferred nanomaterials according to the requirement [27], [28]. After the functionalization, these membranes offered several benefits over the un-functionalized membrane. Hence, functionalization of the membranes surface, pore walls, and deposition of nanostructures is a proficient procedure for gas separation [29]. There are various functionalize groups like hydroxyl, amino, carboxyl, amide, thiol, etc, by which sign of the surface charge can be altered [30]. So, functionalized membrane with deposition required nanomaterials can be used for improvement in hydrogen selectivity [31], [32]. Friebe and Ulbricht [33] have reported that PET membranes are functionalized in a controlled way by surface initiated atom transfer radical polymerization method. In this method, a polymer layer was grafted on the track etched PET membrane. Benjamin et al. have made an effort. by functionalize silica membrane with an amino group by different three synthesis procedure and find that procedure with a maximum loading of the amino group has the highest permeation of CO2 [34]. Functionalized Ca3(PO4)2 nanoparticles were dispersed into the non-functionalized/functionalized track-etched PET membrane by Urch et al. [35]. They observed that no particle detected on the non-functionalized track etched PET membrane while functionalized membranes have more binding of calcium phosphate nanoparticles.
For the metal based hydrogen separation methods, Metallic elements like palladium (Pd) [36], [37], nickel (Ni) [38], copper (Cu) [39], platinum (Pt) [40], silver (Ag) [41], etc., can be chosen because of having excellent capability to separate the hydrogen. Palladium (Pd) is one of the best sensitive catalytic metal which is frequently used for hydrogen separation, hydrogen storage as well as hydrogen sensing [42], [43], [44], [45], [46]. Pd and its alloys are favorable due to its high absorption property for hydrogen. Pd nanocomposite membranes are one of the promising technology because of high permeability and superb separation efficiency for hydrogen [47], [48], [49]. Pd composite membranes have significantly higher permeability and selectivity of hydrogen (H2) than carbon dioxide (CO2) and nitrogen (N2) [44], [50]. Kanezashi et al. [51] observed that Pd-SiO2 membrane of thickness approximately 300 nm were fabricated by sol gel method, in which Pd nanoparticles with having particle size 2–30 nm, were dispersed in a SiO2 layer. They found selectivity of H2/He is 2.2 and H2/N2 are 260 at higher temperature. The permeability of H2 is controlled by the diffusion of hydrogen through the metal lattice [52]. The self-supporting metallic membrane also has good mechanical potency compare to a thin metal layer.
In the recent study by our group observed that as the functionalization time of PET membranes increases than permeability and selectivity improved. But the selectivity approximately gets saturated after 6 h of functionalization. That why in the present study we have used 6-h functionalization time of PET membrane with varying Pd nanoparticles deposition time [53]. Formulation of an ultra-thin Pd membrane is rather challenging and catch the attention of innovative goal for separation technology. Therefore, it is very advantageous to introduce Pd nanoparticles into the pores as well as on the surface of functionalized membranes. Here, our objective of this study is to notice the effect of depositing time of Pd nanoparticles on the permeability and selectivity of H2/CO2 and H2/N2 for the carboxylated PET membranes. So, here we report the interaction of Pd nanoparticles with the carboxylated track etched PET membrane for gas separation. Permeability and selectivity of hydrogen, both are improved as increasing deposition time of Pd nanoparticles. Due to improving permeability and selectivity of H2, these membranes have more potency in hydrogen separation and also promising for the economic use because of the easy formulation procedure and low cost.
Materials and methods
Materials
The Track-etched PET membranes were obtained from Sterlitech Corporation, USA. The average pore diameter of this membrane is 0.2 μm. The chemicals which are used in functionalization process with the carboxylic group were Potassium permanganate (KMnO4, Merck Millipore, India), Sulfuric acid (H2SO4, Sigma-Aldrich, India), Ethanol (C2H6O, Merck Millipore, India), Hydrochloric acid (HCl, Merck Millipore, India). Chemicals which are used for Pd nanoparticles synthesis were Palladium chloride (PdCl2, Sigma-Aldrich, India), Whatman 0.2 μm PVDF syringe filter (GE Healthcare, USA), Trisodium citrate (Na3C6H5O7, Sigma-Aldrich, India) and Sodium borohydride (NaBH4, Sigma-Aldrich, India). Chemicals are used as received form the supplier.
Carboxylation of PET membrane
Functionalization of membranes is one of the expert techniques that can be used to transform the surface properties of the polymer. To the functionalized PET membrane by carboxylic (–COOH) group, we use KMnO4 (5 g) by dissolved in 100 mL of aqueous H2SO4 (0.75 N) solution and stirred carefully for 60 min. to confirm the mixing. Porous PET membranes were immersed in this solution for 6 h to functionalized membranes by the carboxylic group. After the dipping process 6 mol L−1 HCl and C2H6O were used to wash these membranes, respectively and dried at 45°C for overnight. These modified membranes were immersed in Pd nanoparticles solution for a various time like 1, 3, 6 and 12 h. Two times DI water was used to wash these Pd deposited membranes. For the comfortability functionalization time and Pd nanoparticles, deposition time was used to indicate the sample name as shown in Table 1.
Sample name based on the carboxylation and Pd nanoparticles deposition time (c; carboxylated, C–Pd; carboxylated membranes with Pd nanoparticles, numbers represents hours).
| Polymer | Carboxylation time | Pd nanoparticles deposition time | Sample name |
|---|---|---|---|
| PET | 6 h | 0 h | C6–Pd0 |
| PET | 6 h | 1 h | C6–Pd1 |
| PET | 6 h | 3 h | C6–Pd3 |
| PET | 6 h | 6 h | C6–Pd6 |
| PET | 6 h | 12 h | C6–Pd12 |
Synthesis of Pd nanoparticles and Pd nanoparticles deposition
The chemical route was followed to synthesis Pd nanoparticles in solution form. During the synthesis process of Pd nanoparticles, we used palladium (II) chloride, concentrated HCl, sodium borohydride, sodium citrate, DI water and 0.2 μm pore size syringe filter. Entire details of preparation method and synthesis of Pd nanoparticles has been charted in our previous reported work [37], [54]. For the deposition of Pd nanoparticles into track-etched PET membranes, membranes were dipped in the Pd solution for the various time. Information about the deposition time of Pd nanoparticles is given in Table 1.
Permeability measurements
To accomplish the objective of gas separation, we use gas permeability setup, and the details of the setup are given in previously reported work [50]. The gas permeability data for all the gasses are taken within 5–10 psi pressure range. Gas permeability through the polymeric membrane is calculated by using Fick’s formula. Equation (1) shows the Fick’s formula, in which K, d and s were constant for our experimental work.
The symbols in the given equation (1) have their standard meaning. K is cell constant of the gas permeability setup (4.44×10−3), d is corresponding to the thickness of PET membrane, S is defined as movement of mercury slug in U-tube (part of the gas permeability setup), ∆p used for the pressure difference across the sample and t is the time occupied by mercury slug to move a predefined distance.
For gas separation process, highly selectivity membranes are needed and calculated gas selectivity is the ratio of permeability of two different gasses.
Characterization measurements
Spectrum 2 Perkin Elmer setup was to carry out Fourier transform infrared (FTIR) spectra. Spectral range 400–4000 cm−1 was selected for transmission measurements for all the samples. AIRIX STR 500 Raman microscope was used to perform Raman spectroscopy with an excitation wavelength of 532 nm having the power of 12.5 mW and 50X objective lens. Nova Nano 450 scanning electron microscopy (SEM) was used at 15 kV accelerating voltage to examine the surface morphology and dispersion of Pd nanoparticles. Gold coating was used to remove to electron charging effect for polymer sample.
Results and discussion
Permeability and selectivity measurements
The complete gas permeation by a polymeric membrane depends on the following phenomena; (1) absorption of the gas molecule by the polymeric surface of the membrane, (2) diffusion rate of molecule crossover the volume of the membrane sample, (3) the desorption of gas molecule by the outer surface of the membrane. In our study for gas permeation, we use three gasses H2, N2, and CO2. Also, the experimental work carried out at the room temperature, and corresponding results are summarized in Table 2. In the all these gasses H2 (2.89 Å) have a minimum molecular diameter in comparison with CO2 (3.30 Å) and N2 (3.64 Å). The Smaller size of gas molecule always directed more absorption and diffusion through the membrane. Because of this phenomena, H2 shows higher permeability in all samples compared to the other two gasses CO2 and N2. In a comparison of CO2 and N2 permeability, both the gasses shows approximately same permeability at the side of the molecular diameter of CO2 is smaller the N2. Not only the molecular diameter but also the shape and polarity of the molecule affect the permeation of the gas. H2 and N2, both have the spherical shape, but CO2 has the linear molecular shape. Also, the polarity of the CO2 is higher than the N2. Combine result of above effects contributes in the approximately same permeability of the CO2 and N2.
Permeability and selectivity data of all samples (named according to their functionalization and Pd nanoparticles deposition time).
| Sample | Permeability (Barrer) | Selectivity | |||
|---|---|---|---|---|---|
| H2 | N2 | CO2 | H2/N2 | H2/CO2 | |
| C6–Pd0 | 105158 | 51628 | 52372 | 2.04 | 2.01 |
| C6–Pd1 | 112563 | 51231 | 54000 | 2.20 | 2.08 |
| C6–Pd3 | 119284 | 50075 | 49333 | 2.38 | 2.42 |
| C6–Pd6 | 177600 | 55041 | 49950 | 3.23 | 3.56 |
| C6–Pd12 | 222000 | 52031 | 51896 | 4.27 | 4.28 |
Now as Fig. 1 shows that the permeability of hydrogen increases with the deposition time of Pd nanoparticles increase. This enhancement in the permeability is occurred due to the hydrogen absorption property of Palladium metal. As the hydrogen contact with Pd lattice, H2 occupy the octahedral void of the lattice. During the permeability measurement, there will always be concentration gradient of H2 molecule across the Pd nanoparticles. This one-sided hydrogen saturation forced Pd lattice for rapidly diffusion on the other side. Also, Carboxylation of PET membrane improves the attachment of Pd nanoparticles. Because of the carboxylation, active sites on the polymer surface increase compare to the pristine surface. As the deposition time of Pd nanoparticles increases the permeability of H2 improves because of the more no of Pd nanoparticles attachment.
Graph of gas permeability data for all carboxylated PET membranes with various time of Pd nanoparticle deposition time in hours.
Fig. 2 shows the selectivity data of H2/N2 and H2/CO2 for all the samples. According to selectivity analysis, we found that the selectivity improves as the deposition time of Pd nanoparticles increases. The deposition of Pd nanoparticles negligible affects the permeability of CO2 and N2 because of the no interaction with CO2 and N2 molecules. But in the same situation H2 permeability increases. This improvement of permeability drives the higher selectivity with deposition time. The uppermost selectivity for H2/N2 and H2/CO2 was found 4.27 and 4.28, respectively, correspond to the C6-Pd12 sample.
Graph of Selectivity data for all carboxylated PET membranes with various time of Pd nanoparticle deposition time in hours.
FTIR spectra of all Carboxylated PET membranes in the differnt range of wavenumber (a) 400–2000 cm−1 (b) 2000–4000 cm−1.
Fourier transform infrared spectroscopy (FTIR) analysis
In the FTIR spectra analysis of all the functionalized membranes, we observe all the required peaks correspond to the PET polymer are listed in Table 3 and shown in Fig. 3. In the FTIR spectra, 2500–3300 cm−1 is a functional group region. Intensity and boarding of the peak give the quantitative value of functionalization. For the carboxylic group with PET membrane, we found the stretching vibration of O–H bond at 2968 cm−1 in all the samples.
FTIR peaks and their corresponding bonds for all the samples.
| Peak (cm−1) | Corresponding bond |
|---|---|
| 506 | Out-of-plane banding of –C=O bond |
| 724 | Stretching vibration of –C–H (aromatic) |
| 796 | In-plane bending vibration of C–H bond |
| 870 | Stretching vibration of =C–H (aromatic) bond |
| 972 | Out-of-plane banding of –COOH |
| 1020, 1102, 1248 | Stretching vibration of ester bond C–O |
| 1340 | Stretching vibration of –C–H (alkane) bond |
| 1408 | Stretching vibration of C=C (aromatic) bond |
| 1718 | Stretching vibration of carboxyl bond C=O |
| 2912, 2968, 3432 | Stretching vibration of O–H bond |
During the experimental work, we functionalized all PET membranes by the carboxylic group for 6 h. Because of this, there was not any peak intensity, and boarding difference found the carboxylic group peak in all samples. Similarly, there was not any peak difference found the stretching vibration of C=O bond, belonging from the –COOH group at 1718 cm−1. So both the peaks 2968 cm−1 and 1718 cm−1 gives the direction of equal functionalization by the carboxylic group in all the samples.
Raman spectroscopy analysis
Raman spectroscopy was performed to identify the presence of the desired functional group. In the analysis of Raman spectra of Functionalized PET membranes, we found all the essential peak of PET polymer bonds are present. The goal of the Raman spectra is to identify the carboxylic group. Also, our aim is for quantitative comparison of carboxylic group in all samples. All the identified peaks and their corresponding vibration bonds are listed in Table 4. In the comparison study of C=O stretching bond linking with –COOH group, we found that there was not intensity variation of the peak at 1727 cm−1. As Fig. 4 shows Intensity similarity at 1727 cm−1, is an indication for identical functionalization by the carboxylic group in all samples. Raman analysis also makes connections with FTIR results.
Raman peaks and their corresponding bonds for all the samples.
| Peak (cm−1) | Corresponding bond |
|---|---|
| 277 | C–C stretching (ring), CCC bending (ring) |
| 628 | CCC in plane bending |
| 796 | CH out of plane bending (ring) |
| 854 | C−C stretching (ring breathing), C−O stretching |
| 1093 | C–O–C anti symmetric stretching vibration |
| 1112 | CH in plane bending (ring), C−O stretching |
| 1181 | CH in plane bending (ring) |
| 1290 | C–C stretching (ring), C−O stretching |
| 1414 | C–C stretching (ring) |
| 1459 | CH deformation |
| 1612 | C=C stretching (ring) |
| 1727 | C=O stretching |
| 2905, 2911 | C–H Streching vibration |
Raman spectra of all functionalized PET membranes with various time of Pd nanoparticles deposition time.
Scanning electron microscopy (SEM) analysis
To understand the morphological properties, we have performed the SEM technique. Carboxylated PET membranes were studied before and after with the varying deposition time of Pd nanoparticles. By varying the Pd nano particles deposition time, we have arranged all the SEM images in Fig. 5. From Fig. 5 we can conclude that the average pore size is near about 0.2 μm in all the samples. Nanoparticles and polymer surface shows the contrast difference in the SEM images. Bright part in SEM image is an indication for the more Pd nanoparticles. From Fig. 5 we can see that as we increases the Pd nanoparticle deposition time brightness of the pore boundary and surface enhanced. This enhancement in the contrast happen because the more number of Pd nanoparticles attachment with time. Increment in the Pd nanoparticles gives the more selectivity for hydrogen as compare to the CO2 and N2.
SEM images of all the functionalized PET membranes according to Pd nanoparticles deposition time (a) no deposition (b) 1 h (c) 3 h (d) 6 h (e) 12 h.
Conclusions
The performance of track etched PET membrane can be improved through the functionalization with the carboxylic group. Hence, PET membranes functionalized with –COOH group is the proficient way in which permeability and selectivity both were enhanced. PET track etched membrane which is treated with Carboxyl group, dipped in Pd nanoparticle solution. It is concluded from above results that binding of palladium nanoparticles of these membranes are improved with the increment of Pd depositions time. Also, the selectivity of H2/CO2 and H2/N2 both were improved with the increasing dipping time of carboxylated PET membranes in Pd nanoparticles solution. Uppermost selectivity of H2/N2 and H2/CO2 are found 4.27 and 4.28, respectively for 12 h dipped sample. Here, we found more than two times improved the selectivity of highest Pd binding membrane than without Pd embedded membrane. It is assumed that such type of Pd deposited functionalized membranes can frequently be used for gas separation and purification engineering applications.
Article note:
A collection of invited papers based on presentations at the 25th POLYCHAR 2017 World Forum on Advanced Materials Kuala Lumpur, Malaysia, October 9–13, 2017.
Acknowledgments
The authors acknowledge financial support from SERB-DST, New Delhi (ECR/2016/001780) for this research work. Authors KA and MK acknowledges the support of DST-INSPIRE faculty award.
References
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